engine fan stays on

You might have air leaking at the fan hub O-ring that you cannot hear with the engine running. This leakage can vary with change in temp under the hood, possibly leaking when cold and not leaking when under hood temp is warm.

I would do a key on engine off test with full system air pressure first thing in the morning or when you start your day. Open the hood, turn ignition key to on, check fan hub for releasing and for air leak at fan hub. It may take a few attempts to catch it leaking but more likely to leak with cold engine temp.

 

Might be the fan clutch.

 

could be the a/c high pressure fan switch,or harness plug that goes to the switch locate the switch,unplug the harness and use a paper clip or copper wire to bypass the switch.just stick the paper clip into both ends of the harness to continue the the power back through the circuit and see if the fan turns off.i went through the same thing a couple years ago and thats what it ended up being,then a month or so ago my fan would come on for 10 min then shut off then come back on for 1 minute,drove me nuts.ended up being the cat coolant temp sensor.the gauge in dash is mechanical so it uses a different sensor.we had to put on computer to figure out since it wasnt showing high temp readings,when we put on computer the voltage at the cat sensor was erratic so i changed all good now.

 

You can apply air to air line going to fan clutch and see if it works. You can also unplug air solinoid while its running and see if it makes noise if it does.then u know the ecm is trying to release fan.

 

Also check the plugs on the firewall going up to the solenoid on the firewall. Had trouble out of it being corroded on my peterbilt.

 

the intake manifold temp sensor, located just behind the intake pipe ( two wire sensor ) will also cause the fan to stay on. It can stick in range with a hi temp causing the fan to come on trying to cool the intake temp, and not set a check engine light

 

Ive also seen loose connection at coolant temp sensor.

 

this is correct & makes sense. most probable if I was to just guess.